Microphones having a directional characteristic are useful in many applications. One known technique for achieving directionality is through the use of a first-order-gradient (FOG) microphone element which comprises a movable diaphragm with front and back surfaces enclosed within a capsule. The capsule includes openings on each side thereof for admitting sound pressure to interact with the front and back surfaces of the diaphragm. In response to this interaction, an electrical signal is generated that is proportional to the differential sound pressure on the opposite surfaces of the diaphragm. Sounds are ignored that come from directions in which the wave front arrives at the front and back surfaces of the diaphragm at the same time. In this situation the instantaneous sound pressure on each surface of the diaphragm is identical, so the differential sound pressure is zero. Sounds emanating from other directions reach one surface of the diaphragm before the other according to the delay encountered in traveling an effective path length "d" between the opposite surfaces. This delay creates directionality , but it also affects the frequency response characteristic because path length "d" corresponds to a different fraction of a wavelength at each different frequency. Low frequency response is poor because sound waves having practically the same phase arrive on opposite surfaces of the diaphragm at the same time.
U.S. Pat. No. 3,715,500 was issued to Sessler et al. on Feb. 6, 1973 and is entitled Unidirectional Microphones. Sessler et al. disclose the use of tubes in connection with a FOG microphone element to effectively increase the separation between its sound ports. While increased separation improves the low frequency response of the microphone, the disclosed construction is somewhat cumbersome to assemble and install in modern sound-input equipment.
U.S. Pat. No. 4,742,548 was issued to Sessler et al. on May 3, 1988 and is entitled Unidirectional Second Order Gradient Microphone. In this patent, sensitivity is improved by housing a FOG microphone within a baffle so as to increase the effective acoustic path length between sound ports of the FOG microphone. The baffle comprises a flat surface which is preferably square or circular. While this structure represents an advance with respect to the prior art, it does not address the problem of conveniently fitting the FOG microphone into sound-input equipment due to its large physical size and projection above the sound-input equipment surface.
One known microphone device, designated WM-46AAD201, is available from National/Panasonic and provides a cardioid polar response characteristic. A FOG microphone element is enclosed within a rigid, 2-piece plastic housing which includes openings that permit sound waves to enter the housing on each side of the FOG microphone. The housing and the FOG microphone are mutually held together by glue or other bonding material so that each side of the FOG microphone is exclusively influenced by sound waves entering the appropriate opening in the housing. Unfortunately, the construction of such a device is labor intensive due to the use of bonding materials--an extra step that requires curing time. Moreover, if applied improperly, leaks may result, thus changing the acoustic directional characteristic.
One directional microphone assembly of merit which uses tubes to couple a microphone element to the desired sound pickup points is shown at FIG. 2 of the Knowles Electronics, Inc. Technical Bulletin TB-21, "EB Directional Hearing Aid Microphone Application Notes." Unfortunately, no structural means is provided for supporting such an assembly within sound-input equipment, and the tubes do not appear easily sealed against the equipment surface.
Therefore, it is desirable to provide a housing for a microphone element which is of relatively simple construction so that manufacture and installation are facilitated.
Additionally, it is desirable that the microphone assembly retain the functional improvement achieved by Sessler et al. while being unobtrusive in nature and easily installed into sound-input equipment.